Organic light emitting diode (OLED) has the characteristics of low energy consumption, low driving voltage, wide color gamut, simple preparation technology, wide angle of view, fast response, and so on, which is a powerful competitor of the next generation flat panel display device, becoming a hotspot of research in recent years.
The series OLED with extensive research comprises a plurality of light emitting units. The plurality of light emitting units are connected with charge generation layers without mutually affecting their light emitting. Moreover, the current density in the device is relatively low, effectively avoiding the thermal quenching effect caused by excess current, further improving the OLED current efficiency, brightness, life and so on. By doping luminescent materials with different luminescent wavelengths, the light emitting units of the series OLED can produce red, green and blue light simultaneously; therefore, the application of the series structure in the preparation of high performance white organic electroluminescent devices is also very concerned. Moreover, considering the uneven quality of image displaying, compensation can be realized by adding transistors; the best effect can be achieved especially in the top emitting OLED devices.
However, the number of layers in the existing top emitting series OLED is too large, resulting in a high driving voltage for the device; moreover, during the injection of the carrier from the charge generation layer into the light emitting layer, the carrier can reach the light emitting layer only by overcoming the energy level difference between the electrode and injection layer, the energy level difference between the injection layer and transport layer, and the energy level difference between the transport layer and light emitting layer. Therefore, if the series OLED structure is applied into the top emitting device, it is necessary to solve the problem of high driving voltage caused by a great energy level difference. If there is a great energy level difference at the interface, the carrier cannot enter the light emitting layer easily; the carrier will thus accumulate on the interface barrier, resulting in a high driving voltage for the light emitting device, reducing efficiency.